A vast majority of existing large commercial dryers, employed in the drying of foods and other products, employ a temperature determining means in the dryer and rely on automatic control of this parameter alone. The reason is that this is the easiest variable to control.
One problem is that control of temperature in a dryer does not automatically control flow of make-up air to the dryer. As the moisture content in the dryer increases, it is conventional practice to exhaust amounts of dryer air, usually on a continuous basis, and to bring make-up air into the dryer. The amount of desired exhaust flow obviously depends on the amount of evaporation occurring in the dryer. This typically varies during normal operation, and most dryers thus simply operate at excessive exhaust and make-up flows in order to insure that during periods of high evaporation there is sufficient exhaust flow. The amount of exhaust flow and introduction of make-up air is obviously a heat load in a drying oven, and operation at excessive exhaust and make-up flows represents a waste of energy and money. Increasing competitive pressures and unprecedented escalation of energy costs make more than minimal control of the dryer necessary.
In addition, it has been determined that operation of dryers at higher temperatures can result in substantial savings, making it feasible to determine the maximum allowable drying temperature permissible without product degradation, and to operate the dryer as close to product limits as possible. In most dryers, there is an initial constant rate drying period, and particularly during this period, the wet bulb temperature has a very strong influence on material temperature. This variable thus becomes an important consideration, in combination with temperature measurement, as a means for permitting operation at maximum temperature while at the same time insuring that high temperature operation does not result in such product degradation.
Systems are known for determining conditions in a dryer and compensating for changes in the humidity of the drying environment. Prior U.S. Pat. No. 3,259,995, filed by John W. Powischill and assigned to assignee of the present invention, describes a system for automatically controlling drying conditions to compensate for variations in moisture content within a dryer. However, direct measurement of humidity is not involved. In prior U.S. Pat. No. 3,110,442, by Roger K. Taylor, means are provided for obtaining a pressure variation reading in response to increased humidity and introducing fresh air of controlled humidity into a dryer as the pressure reaches a set point.
One problem with direct reading of humidity in dryers is that many commercially available sensors or probes which are capable of measuring humidity are sensitive to elevated temperatures and incapable of use at temperatures above about 220.degree. F. Above about this temperature, damage to the sensors or probes is likely to occur, and commercial dryers operate at much higher temperatures, for instance up to about 800.degree. F. or higher. Systems have been designed that continuously draw a filtered sample from a dryer and cool it to within a permissible probe temperature range, one such system being shown in prior U.S. Pat. No. 2,643,464 to Hadady. However, conventionally such systems are complicated, some being based on dilution of the sample gas with compressed gas, are expensive to buy and difficult to operate. Also, particular care must be taken in cooling the sample gas, as condensation of moisture within the gas and contact of the moisture with the humidity sensor or probe can invalidate the humidity reading.
In prior U.S. Pat. No. 2,987,918 to Hanna, there is shown a system for determining absolute humidity of gases. In the system, two humidity sensing devices are employed with means for cooling separate gas samples transmitted to the two sensing devices. The cooling means are connected in series so that the temperature of one gas sample is lower than or equal to the temperature of the other gas sample. The humidity sensing devices operate on the principle of measuring absolute humidity or dewpoint by a chemical hygrometer, specifically one in which a current flow in a winding is established proportional to humidity. The signal produced is proportional to the equilibrium temperature established in the winding.